Sulphur sensitization by means of labile sulphur compounds is a widely-used method for conferring speed and contrast to a silver halide emulsion. It is normal practice today to start with inert gelatin which is substantially free from labile sulphur compounds and to introduce them, e.g. thiosulphate, in desired quantity during chemical sensitization. After addition to the silver halide emulsion, the sulphur sensitizer is rapidly adsorbed to the crystal surface. Its adsorption may occur either by the reaction of silver ions present on the crystal surface with sensitizer, e.g. thiosulphate, to form the complex ion in adsorbed state or by the formation of the complex ion from the very low concentration of silver ions existing in solution in the emulsion, followed by adsorption to the crystal surface. The adsorbed complex is then decomposed into Ag.sub.2 S.
The silver sulphide forming part of the crystal surface by displacement of bromide ions and the replacement of each pair of such ions by one doubly-charged sulphide ion must not be confused with the massive quantities of silver sulphide formed in a simple chemical reaction wherein silver sulphide is produced as a brown precipitate.